Clell J Solomon

Computational evaluation of neutron multiplicity measurements of polyethylene-reflected plutonium metal

Abstract Simulations of neutron multiplicity measurements of a highly multiplicative plutonium sphere measured with a moderated array of 3He proportional counters have consistently overpredicted the mean and variance of the measured multiplicity distribution. In contrast, identical experiments using a 252Cf source have been accurately simulated. This paper outlines a sensitivity analysis of several key parameters that could account for the overprediction in the simulation of the plutonium sphere. Parameters that were analyzed include source-detector distance, detector dead time, variations in density and volume of the plutonium, and the value of for v̄ 239Pu-induced fission. Of these parameters, the only factor that accounted for the overprediction within reasonable bounds was a change in the value of the 239Pu v̄. The sensitivity analysis showed that a small change (1.14% reduction) in the value of v̄ dramatically improved the simulated results.

A Priori Deterministic Computational-Cost Optimization of Weight-Dependent Variance-Reduction Parameters for Monte Carlo Neutral-Particle Transport

Abstract A method for deterministically minimizing the cost of a single Monte Carlo tally employing weight-dependent weight-window variance reduction has been developed. This method relies on deterministic calculations of the tally’s variance and average computational time per history, the product of which is the cost (inverse figure of merit) of the tally calculation. The tally’s variance is deterministically computed by solving the history-score moment equations that describe the moments of the tally’s score distribution, and the average time per history is computed by solving the future time equation that describes the expected amount of computational time a particle and its progeny require to process to termination. Both equations are solved by the Sn method. Results are presented for one- and two-dimensional problems that demonstrate increased calculation efficiency, by factors of 1.1 to 2, of the optimized problems over standard adjoint (importance) biasing.

Milestone 5431: Chi-Nu Measurements of Prompt Fission Neutron Spectra (PFNS)

This presentation offers an overview of the Chi-Nu project, its results, evaluation, and plans for FY17.

Simulation of Photon energy Spectra Using MISC, SOURCES, MCNP and GADRAS

The detector response functions included in the Gamma Detector Response and Analysis Software (GADRAS) are a valuable resource for simulating radioactive source emission spectra. Application of these response functions to the results of three-dimensional transport calculations is a useful modeling capability. Using a 26.2 kg shell of depleted uranium (DU) as a simple test problem, this work illustrates a method for manipulating current tally results from MCNP into the GAM file format necessary for a practical link to GADRAS detector response functions. MISC (MCNP Intrinsic Source Constructor) and SOURCES 4C were used to develop photon and neutron source terms for subsequent MCNP transport, and the resultant spectrum is shown to be in good agreement with that from GADRAS. A 1 kg DU sphere was also modeled with the method described here and showed similarly encouraging results.